The present invention relates to an apparatus of automatically correcting movement of a specimen three-dimensionally during rotation of the specimen in an electric charged particle beam microscope having a specimen rotation mechanism.
In respective fields of semiconductor device, nano-material, biotechnology and the like, the need for analyzing the structure of a specimen three-dimensionally in an order of nano-meter (nm) has been increasing. Available as an apparatus for observation of s specimen in a nm order are a transmission electron microscope (TEM) and a scanning electron microscope (SEM), by which three-dimensional observation of a specimen structure can be performed by rotating the specimen to observe it in various radial directions.
Japanese Patent No. 3677895 describes, as a technique for three-dimensional observation, an electron microscope for three-dimensional observation and a microscopic method which will be described below. More particularly, a portion to be observed is cut into a projecting shape by using a focused ion beam. The diameter of the projecting portion is set to a value that permits an electron beam to transmit through the portion. The cut-off piece of specimen is fixedly mounted to a uniaxial all-direction rotational specimen holder while making the projection center axis coincident with the specimen rotation axis and the observing objective is observed in all radial directions of uniaxial rotation in a specimen chamber of the electron microscope. Since the observation direction is not limited in the general-use TEM/STEM using thin-film specimens, the specimen structure can be observed three-dimensionally with ease. When restructuring a specimen structure from transmission images obtained through observations in various directions, the specimen structure of less artifact can be restructured.
In three-dimensional observation, a specimen is rotated frequently in order to observe the specimen in various directions. A factor of degrading the TAT (thematic apperception test) in the three-dimensional observation originates from correction of a movement of specimen due to a specimen rotation. In the electron microscope, the view-field diameter is in an order of micrometer (μm) and therefore, it is difficult to fixedly mount a specimen by coinciding the specimen position with the rotation axis exactly with this accuracy. For this reason, as the specimen rotates, the specimen will sometimes move around the rotation axis to deviate from the view-field. To cope with this problem, three kinds of techniques have been disclosed as will be described below.
In the first place, there is a technique for mechanically coinciding the specimen position with the specimen rotation axis. JP-A-2001-312989 gives a description to the following effect. A mechanism is provided which adjusts the direction of the whole of a uni-centrically adjusted stage system to align the rotation axis with the electron microscope center, that is, an adjustment mechanism is provided which includes a second spherical seat capable of adjusting the direction in which the whole of the uni-centrically adjusted stage is mounted to the electron microscope. With this mechanism, the whole of the stage can be rotated on the second spherical seat without disturbing the uni-centric condition of the rotary cylinder and a specimen holder, thus succeeding in setting of alignment of the cylinder rotary axis direction with the electron microscope view-field center.
A second technique is for measuring and recording a specimen movement due to a specimen rotation in advance and adjusting the specimen stage on the basis of the record such that the specimen movement can be cancelled out. JP-A-8-106873 gives a description to the following effect. An amount of positional displacement of a target area (standard specimen) or an observation objective area (actual specimen) on a plane orthogonal to the electron beam optical axis can be detected by comparing electron microscopic images before and after specimen rotation with each other whereas an amount of positional displacement of a target area or an observation objective area in the direction of electron beam optical axis can be detected by measuring a distribution of intensities of Fresnel fringes developing in an electron microscopic image of the specimen. Then, amounts of positional displacements at a rotation angle θ in X-axis, Y-axis and Z-axis directions are determined on the basis of information of the stored positional displacement amounts and position correction signals of values of a sign inverse to the positional displacement amount are generated and forwarded to a specimen moving mechanism control system which in turn corrects the positional displacements.
A third technique is for sequentially measuring and correcting specimen movements due to specimen rotations without resort to the use of records of specimen movements. JP-A-11-288679 gives a description to the following effect. In order to rotate a specimen to an intended direction, a crystal azimuth and the direction and angle of a specimen rotation necessary for directionality from the azimuth toward an intended azimuth of observation are calculated. A specimen rotation mechanism is so controlled as to slightly rotate the specimen to such an extent that an image of the specimen does not deviate from the screen of a display means and the thus developing positional displacement of the specimen is calculated by an analyzing means. As the specimen positional displacement analyzing means, a means for analyzing a positional displacement between images through a mutual correlation method is described. A position correction means controls the specimen moving mechanism and a deflection coil such that the positional displacement can be eliminated, thereby enabling position correction control to be carried out. Then, the specimen rotation and position correction control as above are repeated until the rotation in the intended azimuth can be attained.
Further, the specimen movement measurement can principally be classified into movement measurement in XY plane orthogonal to the electron beam optical axis and movement measurement in Z direction parallel to the electron beam optical axis. In the case of the movement in XY plane, a movement of a specimen is measured by taking images before and after specimen movement and processing the picked up images through a normalized mutual correlation method or a phase limited correlation method. A specimen movement in Z direction is measured from a specimen position in relation to a focal position of the electron lens. Available as a specimen defocus amount measurement method is a method of utilizing parallax and a method of utilizing an image sharpness degree evaluation value. JP-A-2000-331637 gives a description to the effect that a positional displacement amount due to parallax of the electron microscope is detected through image processing and the result is fed back to the electron beam apparatus. JP-A-2001-098048 gives a description purporting that the direction/magnitude of an astigmatic difference and the focal offset are detected by image-processing two-dimensional particle images acquired by changing the focal point and then, they are collectively converted into two kinds of astigmatism correction amount and focus correction amount for execution of correction. In the method as above, about 20 sheets of a series of defocus images are used typically.